In the development of new drugs, one of the important characteristics is the binding of the new chemical entity (NCE) to receptors or other cellular proteins, while binding to plasma proteins or to untargeted cellular components, such as lipid membranes often is an unwanted side-effect and could be detrimental for the pharmaceutical activity.
A drug's efficiency may be affected by the degree to which it binds to untargeted cellular components or to the proteins within blood plasma. The less bound a drug is, the more efficiently it can traverse cell membranes or diffuse. Protein binding can influence the drug's biological half-life in the body. The bound portion may act as a reservoir or depot from which the drug is slowly released as the unbound form. Since the unbound form is being metabolized and/or excreted from the body, the bound fraction will be released in order to maintain equilibrium.
Thus, knowledge about the amount of plasma protein or cellular component binding is essential to predict the behaviour of the drug in the body and to establish a rationale on dose-effect relationships.
Nowadays, measuring binding to plasma proteins is accomplished by an equilibrium dialysis, from which affinity/binding coefficients can be measured (see e.g. Shibukawa, A. et al., 1994, J. Pharm. Sci. 83:868-873; Minagawa, T. et al., 1994, J. Pharm. Pharmacol. 46:838-843; Curry, S. H. et al., in: Analysis of Psychotropic Drugs, Ed: Boulton, A. A. et al., Springer Protocols: Neuromethods, Vol. 10, p. 1-31). However, some NCEs, notably hydrophobic NCEs, have such fysico-chemical properties, which makes them incompatible with the current technologies, that equilibrium dialysis is no longer feasible. The above-mentioned prior art all mention special techniques for equilibrium dialysis techniques to be able to measure the interaction between hydrophobic drugs and plasma proteins.
Thus, there is still need for a simple and reliable method to assay the affinity/binding coefficients of hydrophobic small chemical compounds to proteins or biological, i.e. cellular components.